Suppose you have a three phase grounded neutral Y, and Van phase is V<0 (V at an angle of 0), Vbn is V<-120 (Vbn lags Van by 120 deg) and Vcn is V<-240 (Vcn lags Vbn by 120 deg).
Then if you put your meter between A phase and B phase, you'll measure Vab=Van-Vbn= V<0 - V<-120. If you do the vector math, you'll find that the result is V*sqrt(3)<30, or Vab leads Van by 30 deg.
If you're asking about positive, negative and zero sequences, that's a different problem. Basically, if you have three vectors, say Vab, Vbc, and Vca, rotating counterclockwise in a sequence Vab, Vbc, Vca, if they're not symmetrical (equal in magnitude and angle) then they can be represented by three sets of three symmetrical vectors, the sums of which add up to the original vectors. It's very complicated. I have a good link about it, but it's at work.
j.